>> Would a ratio of loft over total weight of bag give a good figure for comparing sleepingbags?
We looked at doing something similar when we wrote our Unconventional Sleep Systems article. But the problem is the huge difference in size of the various bags. A narrow, Hobbit-sized bag would score unfairly well over a larger one — even if it failed to provide enough coverage to provide adequate warmth.
Cheers,
-Mike

